In general, a motion detector detecting the status of displacement of an object by the Doppler Effect requires a high frequency signal output from an oscillation source operated at a high frequency to update the status of displacement of the object. Many of these motion detectors use a phase lock loop (PLL) to stabilize the high frequency oscillation signal. However, a PLL typically requires one of two implementations. In one method, a frequency divider is used for dividing the high frequency oscillation signal before the divided signal is then compared with a low frequency reference signal by a phase detector. Unfortunately, the frequency divider could consume a great deal of power during operation of the PLL. In an alternate method, a high frequency reference signal is provided for comparison with the high frequency oscillation signal by a phase detector. In this case, although the high frequency oscillation signal is not divided by a frequency divider, the phase detector has to perform phase detection at a fast rate, which could lead to increased power consumption. Therefore, a solution with lower power consumption is required in this field.